In an image display apparatus, such as a liquid crystal display, many liquid crystal pixels are arranged in a matrix pattern, and intensity of transmission and reflection of incident light is controlled in each pixel in accordance with image information to be displayed, so that an image is displayed. This is the same in an organic EL display or the like including organic EL devices in pixels, but the organic EL devices are self-light-emitting devices unlike the liquid crystal pixels. Therefore, the organic EL display has advantages of having higher visibility than that of the liquid crystal display, not requiring a backlight, and having high response speed. Also, the brightness level (gray level) of each light-emitting device can be controlled by a value of current flowing thereto, and the organic EL display, which is a so-called current-control type, is significantly different from the liquid crystal display, which is a voltage-control type.
As the liquid crystal display, the drive system of the organic EL display is classified into a simple matrix system and an active matrix system. The former has a simple structure, but has a problem of being difficult to realize a large high-resolution display. For this reason, the active matrix system is now being developed actively. In this system, a current flowing to a light-emitting device in each pixel circuit is controlled by an active device (typically, a thin film transistor (TFT)) provided in the pixel circuit, and this system is described in Japanese Unexamined Patent Application Publication Nos. 2003-255856, 2003-271095, 2004-133240, 2004-029791, and 2004-093682.
Conventional pixel circuits are placed at parts where scan lines in rows supplying control signals and signal lines in columns supplying video signals cross each other, and each of the pixel circuits includes at least a sampling transistor, a capacitor unit, a drive transistor, and a light-emitting device. The sampling transistor is brought into conduction in response to a control signal supplied from the scan line and samples a video signal supplied from the signal line. The capacitor unit holds an input voltage in accordance with the sampled video signal. The drive transistor supplies an output current during a predetermined light-emitting period in accordance with the input voltage held in the capacitor unit. Typically, the output current has dependency on carrier mobility and a threshold voltage in a channel region of the drive transistor. The light-emitting device emits light at brightness according to the video signal by the output current supplied from the drive transistor.
The drive transistor receives, in its gate, the input voltage held in the capacitor unit and supplies an output current between the source and drain, so as to bring the light-emitting device into conduction. Typically, the emission brightness of the light-emitting device is proportional to the amount of current flowing thereto. Furthermore, the amount of output current supplied from the drive transistor is controlled by a gate voltage, that is, the input voltage written in the capacitor unit. In the conventional pixel circuit, the amount of current supplied to the light-emitting device is controlled by changing the input voltage applied to the gate of the drive transistor in accordance with an input video signal.
Here, an operation characteristic of the drive transistor is expressed by the following expression 1.Ids=(½)μ(W/L)Cox(Vgs−Vth)2  expression 1
In this transistor characteristic expression 1, Ids represents a drain current flowing between the source and drain, and is an output current supplied to the light-emitting device in the pixel circuit. Vgs represents a gate voltage applied to the gate with reference to the source, and is the above-described input voltage in the pixel circuit. Vth is a threshold voltage of the transistor. Also, μ represents the mobility of a semiconductor thin film constituting a channel of the transistor. Additionally, W represents a channel width, L represents a channel length, and Cox represents a gate capacitance. As is clear from the transistor characteristic expression 1, when a TFT operates in a saturation region, if the gate voltage Vgs rises by exceeding the threshold voltage Vth, the TFT is brought into an ON state and the drain current Ids flows. In principle, as indicated by the above transistor characteristic expression 1, a constant gate voltage Vgs allows the same amount of drain current Ids to be constantly supplied to the light-emitting device. Thus, by supplying the same level of video signals to all the pixels constituting a screen, all the pixels emit light at the same brightness, and uniformity of the screen can be surely obtained.
Actually, however, thin film transistors (TFTs) made of semiconductor thin films of polysilicon or the like have variations in a device characteristic. Particularly, the threshold voltage Vth is not constant and varies in each pixel. As is clear from the above transistor characteristic expression 1, if the threshold voltage Vth varies in each drive transistor, the drain current Ids varies and the brightness also varies in each pixel even if the gate voltage Vgs is constant, so that the uniformity of the screen is impaired. Conventionally, a pixel circuit having a function of cancelling variations in threshold voltage of a drive transistor has been developed, which is disclosed in the above-mentioned Japanese Unexamined Patent Application Publication No. 2004-133240, for example.
However, the conventional pixel circuit having the function of cancelling variations in threshold voltage (threshold voltage correcting function) has a complicated structure, which inhibits miniaturization or higher-resolution of pixels. Also, the conventional pixel circuit having the threshold voltage correcting function is inefficient and causes a complicated circuit design. In addition, the conventional pixel circuit having the threshold voltage correcting function causes a decrease in yield because the number of elements provided therein is relatively large.